1. Field of the Invention
The present invention relates to a magnetic resonance tomography apparatus and to a method for generating a magnetic resonance image of a subject, of the type wherein k-space is scanned by different pulse sequences.
2. Description of the Prior Art
In a magnetic resonance tomography apparatus for generating a magnetic resonance image of a subject, such as a slice of the human body, successive pulse sequences of one or more high-frequency pulses and one or more gradient pulses are emitted into the area of the subject to be scanned. The resulting echo signals are scanned and are used for generating a magnetic resonance image of the area. The local space, in which the subject is situated, or in which the part of the subject to be scanned is situated, is scanned in Cartesian coordinates (x, y or x, y, z) in k-space, which is a Fourier space that is inverse to the local space. The parameter k is also referred to as xe2x80x9cnormalized timexe2x80x9d or xe2x80x9cspatial frequencyxe2x80x9d and results from the product of the gyromagnetic relationship with the gradient field (-portion) and the time. When k-space is scanned by successive pulse sequences, the magnetization dependent on the spatial frequency k results as a measurement signal. On the basis of a fast Fourier transformation (FFT), the desired spatial distribution of the magnetization in the region to be scanned results. K-space can be two-dimensionally scanned or three-dimensionally scanned. Given a two-dimensional scanning of k-space in Cartesian coordinates, kx normally indicates the scanning direction, whereas ky refers to the phase coding direction. The basic magnetic field extends in the z-direction. Therefore, k-space is scanned by rows in the kx direction by means of a readout gradient Gx. Modulation of the phase coding gradient Gy enables a step-by-step scanning of k-space in a row by row fashion. The sequence of the scanning of kx rows can vary dependent on the application. The middle region of k-space, i.e., the region having smaller k-values, contains the bits of information for the low spatial frequencies and therefore is controlling for the contrast of the received image. The outer regions of k-space, i.e., the larger k-values, contain the bits of information for the higher spatial frequencies and therefore determine the resolution of the received image.
In order to optimize the two opposing goals of image quality and short measuring time, different proposals have been made. One of the proposals is to utilize different pulse sequences during the scanning of k-pace. This utilization of sequences referred to as hybrid sequences includes the utilization of different pulse sequences, for example, such as FLASH pulses in the central area of k-space and EPI pulses in the outer areas of k-space, as proposed by P. M. Jacob et al. in xe2x80x9cMR-CAT SCAN: Cardiac Imaging with a new Hybrid Approachxe2x80x9d, ISMRM 1999, No. 1306. It has also been proposed to use different pulse sequences of one single pulse sequence type, however, with different parameters, see Hillenbrand et al. in xe2x80x9cMR CAT Scan: A Modular Approach for Hybrid Imagingxe2x80x9d, ESMRMB 1999, No. 261.
A problem during the use of different pulse sequences or heterogenous pulse sequences during the scanning of k-space is that strip artifacts caused by phase jumps and amplitude jumps occur in the reconstructed image. A cause of these artifacts can be discontinuities between scanning steps that do not immediately follow one another and between different segments in the scanning of k-space. FIGS. 3A, 3B and 3C herein show an example. FIG. 3A shows a scan in two-dimensional k-space, whereby the readout direction corresponds to the kx direction and the phase direction to the ky direction, for example. K-space is scanned by two sequences (sequence 1 and sequence 2), with sequence 1 used in the middle region and sequence 2 in the outside regions. The sectional view of FIG. 3B shows the amplitude jumps in the measuring signal given the transition between sequence 1 and sequence 2. Subsequent to the transformation of the received measuring values by means of a fast Fourier transformation in the spatial domain, these jumps lead to strip artifacts in the reconstructed image, as shown in FIG. 3C. The prior art approaches have tried to minimize the jumps occurring as a result of the use of different pulse sequences when k-space is scanned by raw data filters, such as Hanning filters. FIGS. 4A, 4B and 4C show this course of action. FIG. 4A schematically shows the application of a raw data filter to the scanning shown in the FIGS. 3A and 3B. On the basis of the applied raw data filter, the measuring values received during the scan are smoothed with respect to a continuous curve (see FIG. 4B). The Fourier transformation of the smoothed raw data results in the subject profile as shown in FIG. 4C. The disadvantages of the application of raw data filters, such as a considerable loss in image sharpness, are obvious when the FIGS. 4C and 3C are compared.
An object of the present invention therefore is to provide a magnetic resonance tomography apparatus for generating a magnetic resonance image of a subject and to provide a method for generating a magnetic resonance image of a subject, wherein an optimally efficient image quality is achieved despite the utilization of different pulse sequences when k-space is scanned.
This object is achieved in a magnetic resonance tomography apparatus having a gradient field system for generating gradient pulses, a high-frequency system for generating high-frequency pulses and for scanning echo signals, and a control apparatus for driving the gradient field system and the high-frequency system such that k-space corresponding to a region of the subject to be scanned is scanned by successive pulse sequences composed of one or more high-frequency pulses and one or more gradient pulses with at least one first pulse sequence and a second pulse sequence that is different from the first one being used, and wherein a transition area, within which the first pulse sequence is converted step-by-step into the second pulse sequence, is defined between the first pulse sequence and the second pulse sequence.
It has shown that the strip artifacts and jumps in the reconstructed image, which are caused by the utilization of different pulse sequences, can be significantly reduced or even completely prevented by such a step-by-step transition of the one pulse sequence into the other pulse sequence. It is thus possible to optimally adjust the measuring parameters desired for the generation of a magnetic resonance image, such as image quality and measuring time, in a simple way.
The first pulse sequence and the second pulse sequence preferably belong to the same pulse sequence type and in the transition area, the sequence parameters of the first pulse sequence are adapted step-by-step to the sequence parameters of the second pulse sequence. This enables an almost-continuous transition of the first pulse sequence into the second pulse sequence. The sequence parameters, which are adapted step-by-step, are one or more parameters from the parameter group of echo time, repetition time, readout bandwidth, excitation angle etc. For example, the first pulse sequence and the second pulse sequence can be spin echo sequences, with the readout bandwidth of the echo being increased and the echo time and the repetition time being reduced in the transition area from the first pulse sequence to the second pulse sequence. The excitation angle can be simultaneously reduced in the transition area.
Alternatively, the first pulse sequence and the second pulse sequence can belong to different pulse sequence types and in the transition area, the pulse sequence type of the first pulse sequence is adapted step-by-step to the pulse sequence type of the second pulse sequence. An advantage is to calculate the step-by-step transition of the fist pulse sequence into the second pulse sequence on the basis of phase values and amplitude values of the two pulse sequences.